Are there any good oficial rules for designing custom drones? I've been just taking the closest thing to what I want and extrapolating stats from that but I was wondering if there were any official guidelines.

Nope, no official Guidelines. What you have been doing is the only way to go currently.

Either use the SR3 rules and translate it a crudely into SR4 (SR3 HAD rules) or just wing it. TRUST YOURSELF.

I was always fond of the SR3 rigger's black book and associated vehicle design rules.

I miss picking my own power plant. Made my anthroform drones with nitrous seem a little more legit. Also nothing like letting the petrol engine run while you're in the elevator

You can always do modifications of the existing SR4 drones, which is enough for many purposes. Want Rigger 4!